LDCs of the kind described above have a semiconductor body based on gallium nitride (GaN) that is disposed on a silicon carbide (SiC). Such luminescent diodes are fundamentally known, for instance from the book “The Blue Laser Diode” by Shuji Nakamura and Gerhard Fasol, published by Springer Verlag, Berlin and Heidelberg, 1997.
Upon a flip-chip mounting, the LDCs are mounted on a carrier, in particular a submount or leadframe of a package, for instance by means of soldering. This has the advantage of very good transfer of current and heat by way of the metal solder. Moreover, the solder can be applied over a large area, so that the transport of current and heat can take place essentially over the entire layer surface. Thus, particularly in high-power components, mounting LDCs by soldering is fundamentally preferable to other methods, such as adhesive bonding or mounting by way of microscopic bumps and the like.
However, in flip-chip mounting of the LDC by means of liquid solder melts, the problem arises that solder adheres not only as intended to the contacts of the LDC. Instead, a direct contact from the solder to the substrate often results. Since the SiC substrate is doped and conductive, this creates a parasitic contact by way of which current flows past the active luminescent layer in the semiconductor body. Such shunt connections lead to belated onset of the generation of light in the luminescent diode; that is, for the same operating current, the diode is darker than a flaw-free diode without shunt connections. If the shunt connection is very pronounced, in an extreme case it can even lead to complete failure of the component.
If GaN luminescent diodes on SiC substrates are placed on a submount covered with a thin solder layer and heated jointly with it without additional pressure, then for a typical lateral length of the LDC of approximately 250 μm, a typical height of the GaN semiconductor body of approximately 3 μm, and a typical thickness of the solder of approximately 1 to 2 μm, a contact between the SiC substrate and the solder from slight tilting of the LDC or as a result of unevenness can be avoided only with difficulty. In this method, the yield of properly functioning luminescent diodes is unacceptably low.
Although this problem can be circumvented by using nonconductive substrates, such as sapphire substrates, those substrates have still other disadvantages.
One way of dealing with this problem in conductive substrates and achieving an acceptable yield is to draw a dielectric passivation over the side edges of the semiconductor body. As a result of the dielectric passivation, the chip edges and a large part of the substrate surface can be protected.
However, this kind of passivation is applied before the LDCs are cut apart from the wafer composite. To prevent the brittle passivation material from flaking off when the chips are cut apart, the passivation must end at a certain distance from the edge of the chip, so that the chip edges themselves, once the chips have been cut apart, are unprotected. The side faces of the substrate in this method cannot be protected either. To achieve the aforementioned acceptable yield, close tolerance limits for the solder thickness and surface quality of the carrier must therefore be adhered to.
A further problem of luminescent diodes is that these components, compared with other semiconductor components that are flip-chip mounted, such as semiconductor lasers, have relatively small dimensions. The active face occupies a large proportion of the entire chip surface, thus leaving only little available space for any possible corrective measures. Moreover, the luminescent diodes in question here are mass-produced products, which are meant to be produced at low effort and expense.